We used hydrogen peroxide to reproduce gill lesions typical of a broad assemblage of gill diseases encountered in aquaculture, and examined the degree of growth rate depression that it caused. Additionally, we compared growth rates of 60 gill-damaged fish when they were either kept separate from (n=40) or cohabited with (n=20) healthy untreated fish for a period of 8 weeks. In contrast to expectations, treated fish reared separately from controls exhibited a much more dramatic decline in growth rate (30% less than controls; P<0.05) compared with those reared with controls (13% less than controls). Although the effect was transient, and persisted for only 2 weeks, it suggests that when considering the bioenergetic costs of disease, and when designing studies to quantify the costs, the interaction of diseased fish and healthy fish needs to be considered..